Maximiliano Ibarra

α1-Adrenoceptors mediating contraction in arteries of normotensive and spontaneously hypertensive rats are of the α1D or α1A subtypes

Alpha 1-Adrenoceptor subtypes mediating contraction in carotid, aorta, mesenteric and caudal arteries from both Wistar Kyoto (WKY) normotensive and spontaneously hypertensive (SHR) rats were investigated by using the alpha 1A-adrenoceptor agonist methoxamine and antagonized with selective, competitive antagonists WB-4101, 5-methyl urapidil or BMY 7378 (8-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4,5)decane -7,9-dione dihydrochloride). Isometric tension changes were recorded after methoxamine addition to the arterial rings, and the effects of the antagonists determined. All the antagonists shifted to the right the concentration-response curve to methoxamine. pA2 values indicate that all arteries but caudal express the alpha 1D-adrenoceptor subtype, since BMY 7378 values were high in these arteries. Due to the high pA2 values for 5-methyl urapidil and WB-4101 and the low values for BMY 7378 we conclude that the tail artery expresses the alpha 1A and not the alpha 1B subtype. No differences were found between both strains of rats, suggesting that hypertension does not modify the alpha 1-adrenoceptors in conductance arteries.

Functional evidence of α1D-adrenoceptors in the vasculature of young and adult spontaneously hypertensive rats

The role of α1D‐adrenoceptors in the vasculature of spontaneously hypertensive (SHR) and normotensive Wistar Kyoto rats (WKY), of different ages was assessed in pithed rats by the use of the selective α1D‐adrenoceptor antagonist BMY 7378 (8‐[2‐[4‐(2‐methoxyphenyl)‐1‐piperazinyl]‐ethyl]‐8‐azaspiro [4.5]decane‐7,9‐dione dihydrochloride). BMY 7378 displaced the pressor effect of phenylephrine in young pre‐hypertensive pithed SHR rats, but produced no effect in young WKY rats (dose ratio of 3.4 and 1.6, respectively), while in adult rats BMY 7378 produced a greater shift in the phenylephrine response curve than in younger animals (dose ratio of 3.2 and 6.2 in WKY and SHR, respectively). The presence of α1D‐adrenoceptors in the vasculature of pre‐hypertensive rats, suggests its role in the pathogenesis/maintenance of increased blood pressure.

α1D- and α1A-adrenoceptors mediate contraction in rat renal artery

To investigate the alpha 1-adrenoceptor subtype(s) mediating contraction in rat renal artery, we have compared the effect of the alpha 1-adrenoceptor antagonists, 5-methylurapidil, BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl) ethyl) 8-azaspiro (4.5) decane-7,9-dione 2HCl) and chloroethylclonidine on functional responses to noradrenaline. A clear blockade by chloroethylclonidine (10(-4) M) of noradrenaline-induced contraction was observed and, along with this effect. pKB values of 9.12 and 8.40 for BMY 7378 and 9.75 and 10.06 for 5-methylurapidil were obtained, indicating that the renal artery expresses the alpha 1D-adrenoceptor subtype as the one involved in contraction and not only the alpha 1A subtype as has been reported.

Effects of aging and hypertension on learning, memory, and activity in rats

A comparison between behavioral alterations induced by hypertension and aging was made in spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY) of different ages (3-24 months old), trained to perform autoshaping learning and activity tasks. Food-deprived rats received autoshaping training sessions during 6 days; the animals were retrained 1 month later. Two weeks after autoshaping training, the animals were evaluated in the spontaneous activity task during 2 consecutive days. The results show an age-related decrease in learning, memory, and spontaneous activity. Independently of the age group compared, WKY, though showing lower activity, learned and retrieved more than SHR. Accordingly, the reductions in learning and memory were correlated with both aging and hypertension. The combined influence of these two factors had synergistic detrimental effects on cognitive functions.

Differential response to chloroethylclonidine in blood vessels of normotensive and spontaneously hypertensive rats: role of α1D‐ and α1A‐adrenoceptors in contraction

The effects of chloroethylclonidine on α1‐adrenoceptor‐mediated contraction in endothelium‐denuded caudal arteries and aorta from normotensive Wistar and Wistar Kyoto (WKY), and from spontaneously hypertensive (SHR) rats were evaluated. Chloroethylclonidine elicited concentration‐dependent contractions. Maximal contraction was similar in caudal arteries among strains (∼40% of noradrenaline effect). However, chloroethylclonidine elicited a higher contraction in aorta from SHR than from normotensive rats. In Wistar aorta chloroethylclonidine produced the smallest contractile response. In SHR aorta, BMY 7378 and 5‐methylurapidil blocked chloroethylclonidine‐elicited contraction, while (+)‐cyclazocine did not inhibit it; while in caudal arteries, 5‐methylurapidil blocked chloroethylclonidine action; the other antagonists had no effect. In chloroethylclonidine‐treated aorta noradrenaline elicited biphasic contraction‐response curves, indicating a high affinity (pD2, 8.5–7.5) chloroethylclonidine‐sensitive component and a low affinity (pD2, 6.3–5.2) chloroethylclonidine‐insensitive component. The high affinity component was blocked by chloroethylclonidine; while in caudal arteries noradrenaline elicited monophasic contraction‐response curves with pD2 values (6.5–5.7) similar to the low affinity component in aorta. Chloroethylclonidine inhibition of noradrenaline response was greater in aorta than in caudal arteries. Chloroethylclonidine increased the EC50 values of noradrenaline ∼1000 fold in aorta and ∼10 fold in caudal arteries. In SHR aorta BMY 7378 protected α1D‐adrenoceptors and in caudal arteries 5‐methylurapidil protected α1A‐adrenoceptors from chloroethylclonidine alkylation, allowing noradrenaline to elicit contraction. These results show marked strain‐dependent differences in the ability of chloroethylclonidine to contract aorta; moreover, chloroethylclonidine stimulates α1D‐adrenoceptors in aorta and α1A‐adrenoceptors in caudal arteries. The higher contraction observed in aorta from SHR and WKY suggests an augmented number of α1D‐adrenoceptors in these strains.

Pharmacological evidence for interactions between 5-HT1A receptor agonists and subtypes of α1-adrenoceptors on rabbit aorta

This study was designed to determine if alpha 1-adrenoceptors are involved in the vascular responses to 5-HT1A receptor agonists. Buspirone (3.1 x 10(-7)-3.1 x 10(-5) M) and 8-hydroxy-2(di-N-propylamino)tetralin (8-OH-DPAT; 3.1 x 10(-6)-10(-4) M) elicited contractions of rabbit aorta rings which were blocked by prazosin (10(-9)-5.6 x 10(-9) M), but which were unaffected by reserpine pretreatment (1 mg/kg i.p.). 5-Methylurapidil (10(-7) and 10(-6) M) blocked contractions elicited by 8-OH-DPAT and by buspirone, whereas chloroethylchonidine (10(-5) and 10(-4) M) inhibited only the effect of buspirone. In addition, these 5-HT1A receptor agonists relaxed arteries precontracted with alpha-adrenoceptor agonists in a similar range of concentrations in which they elicited contraction. Moreover, 8-OH-DPAT and buspirone protected the alpha-adrenoceptors from the irreversible blockade provoked by phenoxybenzamine (10(-7) M), as judged by the norepinephrine contraction and stimulated phosphatidylinositol labeling. According to these results the contractile and relaxant effects elicited by 5-HT1A receptor agonists are a consequence of a direct interaction with alpha 1-adrenoceptors. The contraction elicited by 8-OH-DPAT may be mediated by alpha 1A-adrenoceptors, whereas both alpha 1A- and alpha 1B-adrenoceptors may mediate the effect of buspirone in rabbit aorta.

Chloroethylclonidine is a partial α1-adrenoceptor agonist in aorta and caudal arteries of the Wistar Kyoto rat

The interaction between chloroethylclonidine (N-beta-chloroethyl-N-methylamino-methyl-clonidine) and alpha1-adrenoceptors mediating contraction in Wistar Kyoto rat arteries was examined. In caudal (alpha1A-adrenoceptors) and aorta (alpha1D-adrenoceptors) arteries, chloroethylclonidine (10(-4) M) elicited contraction with different time frames (maximal effect within 4 min in the caudal artery and at 30-45 min in aorta). Phentolamine (10(-6) M) completely prevented chloroethylclonidine-induced contraction in aorta, but partially did so in the caudal artery. Rauwolscine (10(-7) M) partially prevented the chloroethylclonidine contractile effect in both arteries. Chloroethylclonidine attenuated the contractile effect of low concentrations of norepinephrine, however, maximal contraction was observed at catecholamine concentrations above 10(-7) M in the caudal artery and 10(-6) M in the aorta. It is concluded that chloroethylclonidine interacts with caudal alpha1A-adrenoceptors as an irreversible partial agonist, inducing vascular contraction probably due to Ca2+ mobilisation, and with aorta alpha1D-adrenoceptors as a partial agonist, inducing slow-onset muscular contraction.

The 5-HT2 receptor antagonist, pelanserin, inhibits α1-adrenoceptor-mediated vasoconstriction in vitro

The antagonism by pelanserin (2,4(1H,3H)-quinazolinedione,3-[3-(4-phenyl-1- piperazinyl)-propyl]-HCl), a potent 5-HT2 receptor antagonist, of alpha 1-adrenoceptor-mediated contractions of endothelium-denuded carotid, aorta, mesenteric and caudal arteries of both normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats was investigated. The selective alpha 1-adrenoceptor agonist methoxamine elicited concentration-dependent contractions in all four arterial rings, an effect which was competitively antagonized by pelanserin. pA2 values for pelanserin were in the 7.67-8.11 range when evaluated against the alpha 1-adrenoceptor agonist in arteries from normotensive or hypertensive rats. These data support the conclusion that pelanserin displays alpha 1-adrenoceptor blocking properties. The ability of the 5-HT2 receptor antagonist pelanserin to additionally block alpha 1-adrenoceptor-mediated constriction in different vessels of WKY and SHR may potentially contribute to its blood pressure lowering effects.

Evidence that NAN-190-induced hypotension involves vascular α1-adrenoceptor antagonism in the rat

The effect of NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido]-butyl] piperazine), described as a mixed 5-HT(1A) receptor agonist/antagonist, on cardiovascular function was studied. The i.v. injection of NAN-190 (1-300 micro/kg) dose-dependently decreased blood pressure (p<0.001), while heart rate was not significantly modified compared to saline-treated, anaesthetized adult rats. WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide), a highly selective 5-HT(1A) receptor antagonist, increased NAN-190-induced hypotension (p<0.05). In the pithed rat NAN-190 displaced the phenylephrine dose-pressor response curve to the right; ED(50) values were: approximately 14, 20, 40 and 270 microg/kg for saline and NAN-190 (1, 10 and 100 microg/kg, respectively); similar ED(50) values were obtained with prazosin ( approximately 20, 69 and 358 microg/kg for 1, 10 and 100 microg/kg of prazosin, respectively). NAN-190 shifted to the right the concentration-response curves to phenylephrine in rat tail artery (alpha(1A)-adrenoceptors), in rabbit aorta (alpha(1B)-adrenoceptors) and in rat aorta (alpha(1D)-adrenoceptors), with pA(2) values of 9.47, 9.02 and 9.99; while Schild slopes were -0.78, -1.13 and -0.90, respectively (not significantly different from unity). The results show that NAN-190 induced hypotension in the anaesthetized, adult rat and suggest that this effect could be explained by antagonism of vascular alpha(1)-adrenoceptors.

α1-adrenoceptor blocking properties of spiroxatrine in rat aorta

This preliminary study has analyzed the potential ability of the 5-HT1A ligand spiroxatrine to interact with vascular alpha 1-adrenoceptors. Norepinephrine and the selective alpha 1-adrenoceptor agonist, methoxamine, elicited concentration-dependent contractions of rat aortic rings. In contrast, (+/-)-spiroxatrine (from 10(-8) to 3.1X10(-7) M) was devoid of any effect on vascular tone per se, but shifted the concentration-response curves of norepinephrine and methoxamine to the right in a concentration-dependent manner with pA2 values of 8.48 +/- 0.22 and 8.93 +/- 0.33, respectively. Endothelium removal did not significantly affect the above pA2 values of (+/-)-spiroxatrine. These data, taken in concert, support the contention that (+/-)-spiroxatrine displays alpha 1-adrenoceptor blocking properties in rat aortic rings.